Plasticine would be best if you use a good quality one (good ones are especially made NOT to droop easily under the hot lights often associated with stop motion).
Polymer clays come in various brands and lines which run the gamut from very soft to fairly firm (all can be "leached" to make them a bit firmer), so if you didn't need to use hot lights and weren't in a warm environment, some of the lines could work:
...Kato Polyclay (at Hobby Lobby or online) or FimoClassic
...next in firmness would be Premo, Cernit, and maybe Studio by Sculpey
...then FimoSoft
...and finally SuperSculpey and Sculpey III, with original Sculpey last.
Remember too that the softer polymer clays will also get fingerprints more easily, especially when they're warm.
The brand of plasticine that most animators use is probably the excellent Plastalina by Van Aken.
http://www.vanaken.com/plastalina.html
Van Aken also makes a good plasticine that can be purchased locally at many craft stores called Clay Toons.
http://www.vanaken.com/claytoon.html
(The other plasticines are less good in general, like RoseArt and the others intended only for little kids and which often come in packages of 4 colors rather than being sold by the individual color).
Plasticines are also technically called "modeling clays" but that term is often used loosely to mean any "clay" that can be "modeled" so confusing.
There is a problem though when you mention dressing them with "fabrics."
Both polymer clays and plasticine clays are oil-based so most fabrics would absorb some of the oil from them over time and get darkened ares. (Polymer clays can be hardened though, and then there's no oil for the fabric to absorb but also then not as useful for stop motion.)
You can make the clothing from clay too though:
http://glassattic.com/polymer/sculpting_body_and_tools.htm (click on the category called *Clothing*).
Or you could seal the fabrics with white glues or liquid polymer clay (then bake those) or with something else, or you could use fabrics that aren't absorbent alone, or line other fabrics with those.
And here is some info on stop motion photography which will probably discuss types of clay, lighting techniques, etc:
http://www.google.com/search?q=how+to+stop+motion+animation
http://www.google.com/search?q=how+to+stop+motion+animation+clothing+clay
HTH,
Diane B.